1. Field of the Invention
The present invention relates to an optical filter having two or more reflective polarizers which form birefringent filters with retarders, wherein the energy reflected by the polarizers is absorbed by one or more dichroic elements.
2. Description of the Related Art
Birefringent filters exploit the phenomenon of polarized-light interference. A filter consists of one or more filter stages in optical series, each of which has a retarder, or a network of retarders, which receives light in a known state of polarization and imparts to it a spectrally varying state of polarization; followed by a polarizer which analyzes the light and selects a chosen state of polarization, having a selected spectral response. The polarizer from each stage acts as an entrance polarizer to the next, so an N stage filter has a total of N polarizers, plus one more for an input polarizer if the input light is unpolarized.
Techniques for synthesis of birefringent filters have been described in the literature, as for example in “Optical Network Synthesis Using Birefringent Crystals. I. Synthesis of Lossless Networks of Equal-Length Crystals”, 54 J. Opt. Soc. Am. 1267 (1964), by S. E. Harris, E. O. Amman, I. C. Chang; “Optical Network Synthesis Using Birefringent Crystals. III. Some General Properties of Lossless Birefringent Networks”, 56 J. Opt. Soc. Am. 943 (1966), by E. O. Amman; “Optical Network Synthesis Using Birefringent Crystals. IV. Synthesis of Lossless Double-Pass Networks”, 56 J. Opt. Soc. Am. (7), 952 (1966), by E. O. Amman; and “Synthesis of Optical Birefringent Networks”, Progress in Optics IX 1971, pp. 123-177 (North-Holland, Amsterdam) by E. O. Amman.
Title describes a birefringent filter in U.S. Pat. No. 4,129,357 which differs from Lyot's design in that it uses partially polarizing elements and achieves reduced spectral sidebands compared to a filter that uses perfect polarizers.
Kaye describes a birefringent filter in U.S. Pat. No. 4,394,069 according to Lyot's design, implemented using liquid crystal retarder elements.
Miller teaches the use of liquid crystal elements together with fixed retarders in U.S. Pat. Nos. 4,848,877 and 5,247,378 to form precision tunable birefringent filters. These produce a filter action whose spectral response can be changed electronically.
Birefringent filters have been constructed for use in the visible, the infrared, and the ultraviolet range, with the range of wavelengths being limited primarily by the availability of suitable materials.
Most implementations of the birefringent filter use dichroic sheet polarizer film such as so-called “Polaroid” film as a polarizer. Such polarizers are compact and economical, but have limited spectral range. For example, visible dichroic sheet polarizer does not typically work well in the infrared, where it has limited contrast; or material which has greater contrast in the infrared has reduced transmission in the visible.
Birefringent filters have also been constructed for use in the near-infrared range, using infrared dichroic sheet materials, which are optically lossy; or using dichroic glass material sold by Corning under the trade name Polarcor (Corning, N.Y.). A similar material is offered by Codixx (Barleben, Germany) under the trade name ColorPol. The dichroic glass materials have limited spectral range, outside of which they lose dichroism; also, at short wavelengths they exhibit reduced transmission. Polarizers of this type are much more costly than dichroic film, especially when large aperture is required.
Birefringent filters have been constructed using Wollaston or Rochon prism polarizers. Since these elements are approximately cubic in shape, such a filter is long in the dimension along the optical axis if several polarizers are needed; beyond being bulky, this also limits field-of-view. Such a filter is quite costly and somewhat fragile.
Polarizers have been developed which exploit anisotropic patterned metal films, microscopic in scale, which are sometimes termed wire-grid polarizers. Unlike dichroic sheet or glass, these work by selective reflection so one state of polarization is preferentially transmitted, and the other is preferentially reflected. They are compact, have high contrast and are offered in several spectral ranges. Moxtek (Orem, Utah) offers material of this type under the trade name ProFlux.
Tunable birefringent filters are used together with imaging detectors and control electronics to form multispectral imaging systems. For example, such a system can be made by placing a filter in the beam of an imaging system, prior to the detector; relay lenses may be used to provide a convenient place for the filter in the optical path. One example of such a system is the Nuance imaging system from Cambridge Research & Instrumentation (Woburn, Mass.). Using such a system, one can obtain an image cube, which is a representation of the scene at multiple wavelengths, where a spectrum is available at every point in the image. By unmixing the spectrum into components, one can estimate the presence of various compounds or sources in the scene. Or, one may classify the pixel according to some classification scheme, based on its spectrum and known spectral reference information.